I've paid £350 for a mk2 with no upgrades that was pretty good nick about 3-4 years ago.
I wouldn't pay no more than £400 for an unexpanded mk1 or mk2. The yanks seem to get a bit more twisted over the mk1 and 2 and they tend to prefer the mk1. I don't believe thats really the case over here so prices between mk1 and 2s are pretty similar
Please note the upgrades are expensive. To get my MPC above fully kitted cost almost the same as the MPC! Most of them do usually have at least 1 of the upgrades though and you'll surely be saving money if they are preinstalled rather than buying seperately
http://www.soundonsound.com/readersads/ ... derDatAZ=4
The mk.1 advertised above seems a good bet if it genuinely is mint. I paid £500 for one with the same upgrades about 18 months ago
Don't think you'll be able to plug your headphones into the main outs of a Mk.1 to get round the lack of headphone output. It works but its still too quiet even with the Stereo Mix Volume on max
But why have you got rid of a mk. 2 and on the look out for a mk.1? I presume you had the Mk.2 ages ago?